Our long term objective is to gain a greater understanding of postreplicative DNA modifications, with respect to their nature, mechanism and specificity of occurrence and their role in gene expression. We are primarily interested in normally occurring modifications controlled by cells or viruses (viz. enzymatic processes, rather than chemically-induced modification) since these are more likely to be of biological importance in the growth and development of the organism. Specifically, we are focusing on the DNA-adenine methyltransferase (Dam) of Escherichia coli and phages T2 and T4. We hope to elucidate the domains involved in sequence/substrate recognition and, eventually, the precise mechanism underlying polypeptide-DNA interaction. By site-directed mutagenesis and in vitro recombination, we will attempt to generate enzymes with new sequence specificities. In a separate system (involving phage Mu), we are in the process of defining the various regulatory process by which expression of the DNA modification gene (mom) is controlled. The detailed mechanisms of action of the Mu C (transcription factor), Mu Com (translation factor) and host E. coli Dam methylase (transcription factor) are under investigation.